Transmitting and distributing real-time streaming data such as audio data in a wireless network and playing back the data in real-time, e.g., on a multimedia system that includes network connected loudspeakers, is becoming popular. Such real-time data is generated by a real-time source such as a source of digital audio and/or video data. In such a system, data from a real-time source may be fed into a wireless audio distribution network and played back in real-time on network connected loudspeakers.
For such real-time streaming, such traffic characteristics as throughput, maximum delay, delay and maximum jitter must all be accounted for and balanced in order for the multimedia system to operate correctly. When the multimedia system is running across a wireless network, the delay and jitter introduced due to the unreliable nature of the medium may become severe.
Consider for example a 5.1 surround sound audio system that includes wireless network links to at least some of the loudspeakers, with two front, two rear and one center channel loudspeakers, each having a corresponding wireless channel playback unit. Suppose that the surround sound audio system provides the audio for an audiovisual system, a so-called home theater system that includes video. Suppose further that a real-time source, e.g., a DVD player that provides digital audio output, e.g., in the S/PDIF format—also called IEC958 “Digital audio interface” standard format by the European Broadcasting Union (EBU)—is connected to one of the playback units, e.g., the center channel's playback unit. Suppose further that such a center channel is the source device for data, called a master device herein, and the other wireless loudspeaker subsystems are the recipients, that is, sink devices, called slave devices to the master device herein, and further suppose that the master device controls the multimedia stream to the slave devices. That is, the master device wirelessly distributes the audio to the other channels' playback units each acting as a slave device.
One method of distributing the media data across this kind of wireless network is to use multicast packets addressed to the slave devices. This presents an efficient means to deliver traffic to multiple devices; a single multicast packet is sent on the wireless network to all devices matching the multicast address specification rather than different unicast packets being sent to each individual slave device. Such a method is known to work well, for example, on a relatively reliable medium such as a wired Ethernet.
For wireless distribution, e.g., on a wireless local area network (WLAN), using a single multicast packet may have drawbacks. One possible drawback is that multicast packet delivery to the slave devices, e.g., acting as clients in the network with an access point in the wireless network, is relatively unreliable because multicast transmission does not include an acknowledgement from receiving entities, e.g., the receiving slave devices sending an acknowledgement (an “ACK”).
Furthermore, in an infrastructure WLAN that includes an access point (AP), multicast packets can be delayed significantly by the AP when there are client devices that have power save mode. As is known, in an IEEE 802.11 WLAN, an AP stores multicast and broadcast traffic and delivers such traffic only at particular times after what are called DTIM beacon frames are transmitted by the access point.
Furthermore, multicast packets are typically transmitted at the lowest basic rate of the wireless network, which, in the case of a mixed mode IEEE 802.11b/g network, can be as low as 1 Mbps. This can cause medium saturation even for low bitrate applications. Furthermore, this means significant bandwidth is used for the transmission at such a low rate.
Consequently, multimedia systems running across wireless networks must either not use multicast packets, or must implement higher-layer link reliability mechanisms.
An alternate is to include the master device transmitting unicast packets to each slave device. This has the disadvantage that excessive network bandwidth may need to be consumed. For example, in the example of a 5.1 surround sound system with one master device and four slave devices, four times the amount of traffic would need to be transmitted across the network than in the multicast situation.
Yet another alternate includes using higher-layer retransmission schemes coupled with multicast traffic. This might have the disadvantage of introducing relatively large delays, e.g., in worst-case situations. This further might cause network traffic storms in the case that the wireless medium is disrupted by external interference, so that one or more of the slave devices do not correctly receive a given packet or packet(s).
Thus there is need in the art for methods and systems that combine one or more advantages of multicast transmission, e.g., efficiency, with one or more advantages of unicast transmission e.g., reliability, thus, for example, providing for a balance between network bandwidth usage and transmission reliability.